In recent years, many manufacturers have developed Active Fuel Management (AFM), formerly called Displacement on Demand, systems to improve the fuel economy of internal combustion engines. An AFM engine operates in a normal mode (all cylinders are active) when power above a predetermined threshold is required and in an AFM mode (at least one cylinder is deactivated) when power requirement is reduced. AFM mode produces a higher level of firing force, as a result of increased in-cylinder pressures, for each active cylinder. This higher firing force, in turn, causes torque variations, which produce increased structural vibrations, thereby degrading noise and vibration, N&V, performance. In addition, the AFM mode firing frequency reduces to half of the normal mode firing frequency, resulting in more excitation to structurally sensitive frequency ranges.
In order to cancel engine induced forces, and prevent them from propagating and exciting other structures in the vehicle, various systems have been proposed. One known solution concerns the implementation of passive engine mounts. Conventional passive engine mounts exist in many variations and generally comprise of some combination of mass-spring-damper system optimized to provide dynamic stiffness and isolation at a key vibrational frequency. These systems usually provide less acceptable damping at other frequencies. Therefore, conventional passive approaches of vibration suppression may not meet the N&V requirement for both AFM mode and normal mode of engine operation.
Alternatively, some manufacturers have employed hydraulic-based systems for canceling engine induced forces. An exemplary hydraulic mount assembly presently available combines properties of elastomeric materials with hydraulic fluid, and typically includes a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. The cavity is separated into two chambers by a plate. The chambers are in fluid communication through a relatively large central orifice in the plate. A first or primary chamber is formed between the partition plate and the body. A secondary chamber is formed between the plate and the diaphragm. The conventional hydraulic mount assembly may contain a decoupler positioned in the central orifice of the plate that reciprocates in response to vibrations. The decoupler movements accommodate small volume changes in the two chambers. However, at certain small input vibratory amplitudes and high frequencies, fluid flow between the chambers is substantially avoided and hydraulic damping does not occur. In this manner, the decoupler functions as a passive tuning device. Furthermore, conventional hydraulic-based mounts can be very cumbersome and expensive, as integrated sensors and control hardware must be configured to monitor and respond to the specific frequency of the transmitted force.
Another possible solution to suppress engine induced vibrations is to apply active vibration control systems, wherein an engine mount device includes internal mechanisms to control fluid flow between the chambers in the mount, thus changing dynamic stiffness and other damping characteristics of the mount. In addition, electronic control of the mount is added to be operable to sense vehicle operating conditions, and respond thereto. Generally, active vibration control systems utilize active actuators, such as active engine mounts, to cancel engine induced vibrations, which have a frequency synchronized with the rotational speed of the crankshaft.
One such active engine mount comprises a spring-mass system such as an electromagnetic actuator having an electromagnet and piston. The electromagnetic actuator is electromagnetically driven and operable to generate a neutralizing force in response to forces transmitted to the frame it is mounted on. However, in order to effectively cancel the transmitted or resultant forces, the neutralizing force must be tuned to the amplitude and frequency of the transmitted force. While numerous methods and apparatuses have been developed for generating such a neutralizing force, in all known developments, generating the neutralizing force is achieved independently. That is, independent mechanisms (e.g., additional sensors) are employed in order to tune the frequency of the neutralizing force to the frequency of the resultant forces (which is a function of the rotational speed of the crankshaft). Such independent mechanisms often require expensive and complex control units to effectively cancel engine induced forces.